A system that has proven useful for efficiently producing electricity from the sun's radiation is described in U.S. Pat. No. 4,691,076, which is assigned to the present assignee. In that system, an array is formed of semiconductor spheres, each sphere having a P-type interior and an N-type skin. The plurality of silicon spheres are housed in a pair of aluminum foil members which form the contacts to the P-type and N-type regions. Multiple arrays are interconnected to form a module of solar cell elements for converting sunlight into electricity.
In order to produce efficient quantities of arrays, it is desireable to have a process which is uncomplicated, efficient and cost effective. A key process step in making the solar arrays which has created difficulties is the introduction of controlled quantities of dopant impurity atoms into the silicon spheres of the solar arrays.
In one previously developed method of introducing a phosphorous dopant into the silicon spheres, the dopant is delivered to the surface of the spheres in a vapor-phase at the proper concentration. While this method works relatively well for doping planar silicon surfaces, it tends to be ineffective when applied to spherical bodies, such as those silicon spheres used for solar cells. For example, it is difficult to obtain uniform diffusion depth within the silicon spheres where two spheres have a point-to-point contact. At these point-to-point contacts, the spheres are shielded from the dopant vapor which results in a nondiffused area and causes electrical shorts in the spheres. Consequently, the entire sphere surface must be doped. Additionally, electrostatic problems can result in the vapor-phase to cause silicon spheres to cluster together which causes nonuniform diffusion.
Some experimentation has been performed in the application of a liquid dopant source such as a liquid phosphorous. Generally, this method has been referred to as a spin-on phosphorous dopant which uses a liquid to attempt to evenly coat the silicon sphere. Unfortunately, when using a liquid dopant, it has been very difficult to apply a uniform coating due to the point-to-point contact between the silicon spheres. It has been found that if the liquid is too thin, the silicon spheres tend to form pinholes along its surface. Conversely, if the liquid dopant is too thick, the dopant film on the silicon spheres tends to crack.
Therefore, there is a need for a method which evenly dopes a silicon sphere to create a uniform diffusion along the entire surface of the silicon sphere. Additionally, a need has arisen for a method of doping which will eliminate prior problems associated with the point-to-point contact of silicon spheres. There is also a need for a method which will reduce the number of processing steps associated with phosphorous doping of silicon spheres. Finally, a need exists for a method of doping a plurality of silicon spheres used in solar cells which is cost effective.